This invention relates to semiconductor devices. More particularly, it is concerned with junction field effect transistors of the static induction type and to methods of fabrication.
The static induction transistor is a field effect semiconductor device capable of operation at relatively high frequency and power. The transistors are characterized by a short, high resistivity semiconductor channel region which may be controllably depleted of carriers. The current-voltage characteristics of the static induction transistor are generally similar to those of a vacuum tube triode. These devices are described by Nishizawa et al. in U.S. Pat. No. 3,828,230 issued Aug. 6, 1974, and in U.S. Pat. No. 4,199,771 issued Apr. 22, 1980. See also an article entitled "High Performance Microwave Static Induction Transistors" by Cogan et al published in the proceedings of the International Eectron Devices Meeting (IEEE) Dec. 5, 6, and 7, 1983, Washington, D.C., Paper 9.5, Page 221.
The static induction transistor generally uses vertical geometry with source and drain electrodes placed on opposite sides of a thin, high resistivity layer of one conductivity type. Gate regions of opposite conductivity type are positioned in the high resistivity layer on opposite sides of the source. During operation a reverse bias is applied between the gate region and the remainder of the high resistivity layer causing a depletion region to extend into the channel region below the source. As the magnitude of the reverse bias is varied, the source-drain current and voltage derived from an attached energy source will also vary.
One factor which limits the operation of static induction transistors at high frequency is gate capacitance. In order to obtain high voltage gain the gate junction must be located in the immediate vicinity of the source. A very short gate-to-source distance, however, causes the gate capacitance to be high. In addition, a short gate-to-source distance requires extremely tight tolerances and process control during device fabrication.